Simulation Results.

Many thanks to Dimitri Danyuk for permission to include these
output stage simulations.Here the open-loop linearity of the output
stage is being investigated.

An effect not obvious from the measurements is clearly revealed in this
simulation as a small peak in the gain at the point where the lower half cuts
off. This appears to be caused by the current through Q4 not being compensated
for accurately by the error amplifier because it only passes through half the
output resistor. Normally Q4 current is a small proportion of the output
current, but Q1 cuts off before Q4 and then current from Q4 alone reaches the
load and is not fully cancelled by the error amplifier. Simply increasing R4
from 47ohms to 150ohms reduces the peak as in the lower graph. Unfortunately
there are other effects when this resistor is increased, one of which is that
the base-emitter voltage of Q4 is reduced, and so the quiescent current
increases. Resetting the current to its original value the large signal
distortion is then found to have increased. This is because the base-emitter
voltage of Q4 now varies over a larger range, and the extra error resulting
from this causes the error amplifier to switch off sooner as it tries to
compensate. Small signal distortion is reduced a little, but high signal
distortion is increased more. If the increase in R4 is considered worthwhile
the quiescent current Iq needs increasing to keep large signal distortion
close to the original level. The improvement in small signal distortion at
20kHz with R4 increased is visible on the extracted distortion waveform, a
sharp edge of the wave becoming more rounded, but there is no obvious change
in the total amplitude. A value of 100ohms for R4 seems to give almost the
same visible improvement as 150ohms, so this value seems a good compromise,
with Iq increased to 120mA.

The step in the gain as the error amplifier cuts off looks bad, but only a
small range of gain is shown in the diagram, and actually it is less than a
0.5% change in the slope of the output vs input graph, not a step in the
output voltage. With an 8ohm load instead of 4ohms the effect will be smaller
and occur only at a higher output voltage. To put the gain step in context a
standard class-B amplifier using typical 5% tolerance 0.25 ohm output
resistors could have a similar step in gain purely from the resistors having
different values within their tolerance, this in addition to the usual
crossover nonlinearity, and occuring at small signal levels rather than as
here at 3 amps output.

According to the simulation an increase in quiescent current to 600mA will
ensure that the error amplifier does not switch off even with the 4ohm load,
but of course an efficient heatsink is then needed. An alternative approach is
to adjust the values of the two 0.25ohm resistors. Reducing R2, e.g. by about
10%, will increase the negative signal output current from the lower half. If
the current is then too high the error amplifier will conduct more and add
positive current to compensate. If R2 is reduced R1 should be reduced by the
same amount to keep R1 = R2 + R3. Initial experiments suggest that this method
does work, and a resistor of 2R7 in parallel with R2 plus another 2R7 in
parallel with one of the two 0R25 making up R1 prevented the error amplifier
switching off. One unfortunate effect, not present in the unmodified
amplifier, is that with moderate clipping a serious latch-up effect was
observed. It may be that this is only a problem for this particular design,
and with a more conventional input and driver it will be better behaved, so
this method may still be possible, but for the present design it is not
recommended without further investigation.

Without this modification, and with Iq = 120mA, the distortion at high
signal levels appears to be mostly second harmonic, with third harmonic rising
only near clipping, as would be expected. Small-signal linearity is comparable
to a good class-A design, and a little second harmonic at higher levels (about
0.014% at 20kHz at 3dB below clipping) is nothing to worry about. My
conclusion from the test results and simulations is that some further
improvement may be possible, but even so the result is already significantly
better than would be expected from a typical class-B or class-AB design of
similar complexity.